—, hmmm, that is an important note. The app just can't be blank. Another requirement is that you can't submit the same (or similar) app twice.
Amazingly, if past app challenges are an indicator, then Microsoft doesn't reach that number limit (or they just ignore it). I've submitted apps on the last day for their other app challenges and have gotten the prizes.
And, here's the judging criteria, it really isn't that strict. (For those that have a Windows 8 computer, you can click this link for app examples: ms-windows-store:Publisher?name=Simply%20Advanced)
TLDR: To ensure 5 out of 10 points: get 3 points for good app description + screenshots, 3 points for "interesting feature" or multiple levels, 3 points for good UI by adding a background image or animations, 1 point for sound. Well, at least that is how I read it, the following is straight from the T&C
[quote:395jusr5]# Criterion Description Score
1. Features and/or Levels: Does the app engage the user with a complete and polished experience? E.g. Interesting features to your app that enhance the user experience. Increasingly challenging levels within your game. 30%
2. Usage of sound, music and effects: Does the app use sound, music and animation as appropriate to enhance the user experience. 10%
3. Look and Feel of your App: What is the overall look and feel of your app? Is your app’s UI polished and visually appealing? Did you use background images? Did you add animations to create a sense of fluidity and continuity in the app? Is the UI responsive? 30%
4. Marketing How marketable is your app? Does your app description provide users with detailed information about the app? (This includes multiple screenshots in the app description.) Do your app snap shots showcase the true app experience? Is your app easily marketable? 30%